Parallel plate transmission line antenna



PARALLEL PLATE TRANSMISSION LINE ANTENNA Robert M. Sprague, Waban, Mass., assignor to Andrew Alford, 299 Atlantic Ave, Boston, Mass.

Filed Nov. 4, 1955, Ser. No. 544,924

13 Claims. (Cl. 343-727) The present invention relates to directive radiation of radio high frequency waves using a carrier wave and side bands to provide a radiation for accurately determining direction. This invention incorporates principles embodied in the invention disclosed and claimed in the copending application of Gerald J. Adams, entitled Omnidirectional Vertically Polarized Antenna, Serial No. 650,920, filed April 5, 1957, and assigned to the assignee of this application.

The antenna of the present invention is constructed to radiate a pattern in the shape of a limacon and a second pattern in combination with this to produce equally spaced maxima and minima about the Whole azimuth.

The antenna of the present invention is preferably rotated to produce amplitude modulation of the carrier to any desired depth. By virtue of the character of the side band patterns, an extremely accurate direction determination may be made. As the antenna is rotated, the direction is determined with regard to a particular reference point established in synchronism with the antenna rotation so that in eiiect the orientation of the antenna is known at all times. This does not form a part of the present invention, but uses means generally employed in the art particularly known as Tacan.

In the present invention there are preferably two circular disc lines parallel to each other for producing omnidirectional radiation of the carrier wave. Each of the disc lines is distorted to produce a limacon or heartshaped pattern; and in addition, a multilobed pattern is obtained by the use of spaced dipoles positioned at equal angular spacings about the circular disc lines. Rotation of the antenna will then produce modulation at the frequency of rotation for the limacon pattern and at the frequency of rotation times the number of lobes for the multilobe pattern; that is, if the frequency of rotation of the antenna is 15 r.p.s., then there will be one side band of 15 r.p.s. due to the limacon. If there are 9 lobes in the multilobe pattern, there will be 9 X15 c.p.s. modulation for the multilobe pattern or 135 c.p.s. These two patterns will be combined to produce 18 accurately determinable maxima and minima about the axis of rotation of the antenna.

According to the present invention, relative modulation depths may be controlled to divide the radiation as desired between the multilobe pattern and the carrier pattern.

The invention will be more fully described in connection with the drawings annexed hereto in which:

Figure 1 shows a vertical section through the antenna, and

Figure 2 shows a plan view as viewed from the top of Figure 1.

In Figure 1, is a cross-sectional drawing showing the major components of the present antenna. The radiating portion of the antenna consists of two circular disc linesA and B parallel to each other and separated approximately one-half wavelength. The upper disc line consists of circular parallel plates 1 and 2, and the lower disc line of tent circular parallel plates 3 and 4. A TEM wave is introduced at the center of each of these disc lines and propagates radially the entire length of the line. In-phase radiation takes place from the cylindrical strip-like apertures at the outside edges of each of these lines. This radiation is vertically polarized, and is omnidirectional in azimuth when the axis of the antenna is vertical. The major lobe of the vertical pattern is in a horizontal plane. The radiation in the vertical plane is somewhat similar to that of two dipoles, one above the other, separated a distance of one-half wavelength.

The two disc lines are excited with equal in-phase energy by means of a balun. A coaxial transmission line, consisting of an outer conductor 5 and an inner conductor 6, is passed through the center of the lower disc 4, with the outer conductor making contact with the disc. This line terminates slightly short of midway between discs 1 and 4 and forms the lower half of the balun. Ex tending downward from the center of disc 1 is a cylindrical conductor 7 of the same outside diameter as piece 5. This conductor terminates slightly short of midway between discs 1 and 4.

When the inner conductor 6 of the coaxial feed line is terminated on the face of piece 7, a symmetrical balun results. This balun is enclosed by a metal cylinder 8. The field existing across the gap between pieces 5 and 7 then splits and travels along two coaxial transmission lines. The first line consists of part 7 as inner conductor and part 8 as outer conductor. The second line consists of part 5 as inner conductor and part 8 as outer conductor. As these fields enter the space between (a) discs 1 and 2, and between (b) discs 3 and 4, similar TEM waves are set up in each of these two disc lines which propagate radially, and are radiated at the circular apertures of these lines.

The vertical radiation pattern of the two apertures is affected to a large extent by the presence or absence of a cylindrical vertical ground sheet above and below each aperture. The vertical lengths of these ground sheets are also determining factors on the patterns, inasmuch as currents are induced in them. Four such ground sheets 9, 10, 11 and 12 are shown in Figure 1. The length of each of the sheets 10 and 11 cannot exceed one-quarter wave, since the apertures are separated by approximately onehalf wave. Best vertical patterns from the point of view of small minor lobes and other desirable pattern characteristics have been obtained with ground sheets9 and 12 also being one-quarter wave in length. The gap 17 is preferably made approximately wave deep. This gap makes it possible for potentials of opposite sign to exist on opposite edges. The voltage distribution on the cylindrical sheets is substantially as indicated by curves 18, 19 and by the plus and minus signs on Figure 1. If the top of sheet 9 is assumed to be positive, then the bottom of sheet 10 will be negative. Likewise, the top of the sheet 11 will be positive and the bottom of sheet 12 negative. Similar voltage distributions then exist on sheets 910 and 11-12. This distribution is illustrated on the curves to the left of the figure, and may, to some extent, be likened to the distribution along two thin wire adjacent coaxial in-phase dipoles. The vertical plane patterns of this antenna are quite similar to such a thin wire array.

The antenna of Figure 1 may be considered as radiating three separate signals to produce data for azimuth determination. The first of these is a carrier. The second is a side band pair to produce 15' c.p.s. modulation. The third is another sideband pair to produce cycle modulation.

The antenna thus far described radiates a pure vertically polarized pattern, omnidirectional in azimuth. Vertical plane patterns are controllable to a large extent by choice of (a) the spacing between the two apertures, and (b) by the height of the several cylindrical sheets. The signal radiated in azimuth equally serves as the carrier. When the antenna is rotated at 15 r.p.s. (900 'r.p.m.), no change in carrier signal results. However, if the carrier pattern is distorted to produce a limacon, and the antenna is rotated at 15 -r.p.s., a sideband pair will be created to produce 15 cycle amplitude modulation of the carrier of any desired depth. Furthermore, if the carrier pattern is further distorted to produce a nine lobed pattern, and the antenna is again rotated at 15 r.p.s., a sideband pair will be created and will result in 135 cycle amplitude modulation of the carrier.

With the disc line antenna, limacon distortion (15 cycle modulation) can be easily produced. Across each disc line from .1 to .25 wavelength from the center of the lines, is inserted a piece 13 of metal or a piece of dielectric. These inserts are placed on the corresponding radials, one directly above the other, in order to produce the similar effect on each line. A metal insert 13 tends to reflect a portion of the energy back toward the center of the disc line and out the opposite side of the line. Thus, a lesser amount of energy will be radiated from that part of the aperture nearest the reflector 13, and a greater amount from that part of the aperture away from the reflector 13. The distribution of energy around the aperture is substantially sinusoidal. The insert 13 preferably comprises a thin metal rod inserted in each disc line approximately .15 wavelength from the center of each disc. These rods make contact with both the top and bottom discs of each line and produce about 20% modulation at 15 c.p.s. The rods are light in weight and are placed so near the center of the discs, that they do not materially add to the problem of dynamic balancing of the antenna for rotational speed of, say. 900 rpm. An additional advantage in using this method of producing a limacon is the fact that the 15 cycle modulation depth is constant over all vertical angles. Since the source of reflection is close to the center of the disc line, and relatively far removed from the radiating apertures, only the distributing of voltage around the periphery of the aperture determines the radiated field. The distribution in azimuth of this radiated field is independent of vertical angle.

The third signal radiated by the antenna is the sideband pair producing 135 cycle modulation. This signal is the result of distorting the carrier to produce a nine lobed pattern which, upon rotation at 900 r.p.rn., gives the desired modulation. This result is achieved by radiating another field pattern from an entirely separate source. This source is in the form of eighteen vertical dipoles such as 14 and 15, equally spaced at 20 degree intervals around the antenna directly in front of the central gap 17. It will be remembered that a difier'ence of potential exists across this gap. Therefore, balanced transmission lines connecting the gap opening'with the dipoles will transmit energy to the dipoles, which in turn will be radiated. These dipoles are spaced radially approximately one-quarter wave from the cylindrical sheet. The dipoles radiate only vertically polarized radiation. Nine alternate dipoles such as 1 14', etc., are fed in phase with each other. The remaining nine alternate dipoles, offset 20 degrees from the first set are fed in phase opposition to the first set of dipoles. This phase reversal is accomplished by a 180 degree twist in the transmission lines 16. The eighteen dipoles then radiate two types of signals. The first signal is due to parasitic re-radiation of the carrier as a result of the dipoles being in the carrier field. For this re-radiation all dipoles are in phase and produce an omni-directional pattern. The contribution of the dipoles acting parasitically is a slight modification of the phase and of the amplitude of the carrier pattern. The second signal radiated by the dipoles is that due to energy carried from the gap 17 over the transmission lines to the dipoles. This signal is radiated in the form of an 18-lobed pattern, with nulls between the lobes. The field in alternate lobes is reversed in phase by 180 degrees. The lobe structure is essentially constant over all vertical angles. Only magnitude of radiated field changes with vertical angle.

The radiation from the dipoles is superimposed upon the limacon pattern. If the carrier pattern is in phase with one set of 9 alternate lobes, and 180 degrees out of phase with the other 9 lobes, nine cycle modulation of the carrier takes place. The carrier field will be increased when added to lobes with which it is in phase, and decreased when added to lobes with which it is out of phase. The depth of modulation is determined by the relative magnitude of the fields radiated in the forms of the limacon and 18-1obe patterns. If the phases as well as ratios of the fields in both patterns could be maintained constant over all vertical angles, constant cycle modulation over a wide frequency band would resuit from the horizon to the zenith. Such ideal con ditlcns are difficult to obtain, but due to the various parameters present in the antenna, an approach to the ideal is possible. Each dipole in itself radiates approximately a cosine pattern in the vertical plane. But when alternate out-of-phase dipoles are arranged in a circle around a cylinder, the field decreases much more rapidly as the vertical angle is increased. Therefore, the carrier pattern must be caused to decrease much faster with vertical angle than would be the case with a single aperture source in order to maintain constant modulation depth. With a double aperture carrier source and by properly proportioning the cylindrical sheets, it has been possible to maintain satisfactory modulation from the horizon up to a vertical angle around plus or minus 45 degrees over about 29% frequency range.

The 135 cycle modulation depth is varied by changing the field supplied by the dipoles. The dipoles are coupled to the gap by the transmission lines such as 16. Bemg approximately one-quarter wave in length, these lines act as impedance transformers. Therefore, power radiated by the dipoles can be controlled by varying the characteristic impedances of the transmission line.

While Figures 1 and 2 show circular disc lines, regular polygons could be used in their place. If a regular polygon of 20 degrees per face were used, a dipole or other individual radiator could be positioned in front of each face and connected across the whole gap. In

such a case sheets would be constructed as a regular polygon cylinder.

For the purposes of impedance matching and/or for the purposes of rigidity, the parallel disc lines in Figure 1 may be replaced by conical transmission lines.

Having now described my invention, I claim:

1. Means for propagating a vertically polarized directed radiation of high frequency radio waves comprising at least one circular disc line having opposed spaced plates with an impedance element connected across the.

disc line nearer the center of the plates than the edges for distorting said radiation, means for establishing vertically polarized electromagnetic waves within said circular disc line traveling radially outward, and a plurality of vertically positioned dipole radiators spaced about the circular disc line for producing a directive and multilobed radiation pattern with the lobes of said pattern extending radially from the circular disc line.

2. Means for propagating a vertically polarized directed radiation of high frequency radio waves comprising a plurality of parallelly spaced circular disc parallel plate transmission lines fed at the center, means for establishing vertically polarized electromagnetic waves within said transmission lines traveling radially outward, an impedance across each of said lines nearer the center of the discs than the edges for distorting said radiation and means independent of the disc lines providing a plurality ofv vertically positioned dipole radiators spaced about the circular disc line for producing a directive lobed radiation pattern with the lobes extending radially from the circular disc line.

3. Means for propagating a directed radiation of high frequency radio waves comprising a plurality of parallel plate disc transmission lines spaced parallelly a half wavelength apart at a frequency within the transmission range of the carrier frequency, a choke gap between and parallel to said disc lines and a plurality of dipoles with alternate dipoles having alternate dipole elements connected to opposite sides of the choke gap.

4. Means for propagating a directed radiation of high frequency radio waves comprising a plurality of parallel circular disc transmission lines, an impedance across the disc lines at a point to produce a radiation pattern having the form of a limacon, a choke gap formed between the lines and a plurality of dipoles peripherally positioned outside of said line with means providing alternate dipoles with a phase reversal connection across the gap.

5. Means for propagating a directed radiation of high frequency radio waves comprising a plurality of parallel circular disc transmission lines having ground sheets extending at an angle from the plane of the disc lines at the edges thereof, an impedance across the disc lines at a point to produce a radiation pattern having the form of a limacon, a choke gap formed between successive disc lines and a plurality of dipoles positioned just outside of the ground sheets of the line with means providing alternate dipoles connected in phase reversal across the gap.

6. Means for propagating a directed radiation of high frequency radio waves comprising at least two pair of parallel circular disc transmission lines spaced substantially in planes one-half wavelength apart at the frequency of the carrier wave applied to said disc lines, a choke gap positioned between the disc lines and a plurality of dipoles connected at spaced positions around the choke gap with the dipoles alternately connected in phase reversal across the gap.

7. Means for propagating a directed radiation of high frequency radio waves comprising at least two pair of parallel circular disc transmission lines spaced substantially in planes one-half wavelength apart at the frequency of the carrier wave applied to the disc lines, means for distorting energy propagated in each of said lines to form a radiation pattern of a limacon, a choke gap positioned half way between the disc lines and a plurality of dipoles connected at spaced positions around the choke gap with the dipoles alternately connected in phase reversal across the gap.

8. An antenna for propagating a directed radiation of high frequency radio waves comprising at least two pair of parallel spaced plates each pair forming a circular disc transmission line with said lines in planes one half wavelength apart at the mean frequency of the carrier wave applied to the disc lines, feed means including coaxial transmission means for feeding said lines with vertically polarized energy at the center thereof, an impedance across each disc line at a point closer to the 6 center than the edge thereof to produce a radiation pattern having the form of a limacon, a choke gap positioned between disc lines, a plurality of vertical dipoles radially arranged about and connected across said choke gap with the alternate dipoles connected in phase reversal.

9. An antenna as set forth in claim 8 wherein ground sheets extend from the periphery of each plate in a plane normal thereto.

10. An antenna as set forth in claim 9 wherein said ground sheets are coplanar and said choke gap is positioned between adjacent ground sheets.

11 An antenna as set forth in claim 10 wherein said feed means includes a coaxial line concentric with said disc lines terminating in a symmetric balun, and a plurality of coaxial transmission lines coaxial and in part concentric with said first mentioned line interconnecting said balun and each of said disc lines.

12. Apparatus for radiating high frequency energy comprising, first and second generally parallel conducting plates forming a parallel plate transmission line, a coaxial transmission line having inner and outer conductors with its axis substantially perpendicularly intersecting said plates, a conducting cylinder axially spaced from said coaxial line connecting said inner conductor to said first plate through an opening in said second plate and cooperating with said outer conductor to define an insulating gap therebetween, a hollow conducting cylinder surrounding said outer conductor and said cylinder and contacting said second plate about said opening, third and fourth generally parallel conducting plates generally parallel to said first and second plates forming a second parallel plate transmission line, said outer conductor passing through openings in said third and fourth plates insulatedly separated from said third plate and in conductive contact with said fourth plate, said hollow conducting cylinder contacting said third plate about said third plate opening, a plurality of two conductor radiating elements about the outside perimeter of said transmission lines, one conductor of each of said elements being connected to said second plate, the other conductorbeing connected to said third plate.

13. Apparatus in accordance with claim 12 wherein alternate ones of said one and said other conductors are located on opposite sides of the plane passing through said insulating gap.

References Cited in the file of this patent UNITED STATES PATENTS 2,324,548 Wheeler July 20, 1943 2,508,084 Alford May 16, 1950 2,631,237 Sichak et al., Mar. 10, 1953 2,677,766 Litchford May 4, 1954 2,695,405 Litchford' Nov. 23, 1954 2,760,192 Shanklin Aug. 21, 1956 FOREIGN PATENTS 72,904 Netherlands Aug. 15, 1953- 694, 549 Great Britain July 22, 1953 

